If there is a business rule stating that a Customer object must be associated with every
SalesTransaction, it can BEST be enforced by:

A.
writing the customer creation code so that it requires a SalesTransaction parameter.

B.
writing our object creation code so that it is not possible to create a SalesTransaction object
without its required association with Customer in place.

C.
writing a test case based on the use case requirement.

D.
drawing an association between SalesTransaction and Customer on the class diagram.